If I was rearrested and bailed out but the second bail was returned to me because I wasn't supposed to be re-arrested, is this an unlawful arrest?

It probably does not matter if it is an illegal arrest. An illegal arrest does not block a prosecution. When an illegal arrest is found, the remedy is to suppress evidence seized as a result of the illegal arrest. So, if when you were rearrested, they found something illegal in your pocket, or you confessed to a crime, that is subject to suppression from evidence at your trial (if you have a trial). If they did not gather important evidence as a result of the rearrest, there is no worthwhile motion you can file. Theoretically, you could file a lawsuit against the police for the illegal arrest, but this has some problems: (1) the amount of damages would be small, meaning you could not induce an attorney to take the case for a percentage of the recovery, and (2) the officers could use the "good faith" defense, because if there was no intention to violate your rights, you could not win any recovery. I recommend that you turn your attention to the original charges that formed the basis of the arrest and rearrest, and try to beat those charges, or make an acceptable plea bargain. Good luck.